Previously proposed aircraft collision avoidance systems suffer from many serious disadvantages, mainly because they require the use of a large amount of expensive and cumbersome special equipment and do not take adequate advantage of the standard ground-based navigational equipment now in widespread use and at known locations. Such standard ground-based equipment includes existing navigational aids, known as "very-high-frequency omnidirectional range-distance measuring equipment" (VOR-DME) or comparable ground-based aids to navigation. The locations of these VOR-DME facilities are known, and they are commonly used, employing suitable conventional utilization apparatus on the aircraft.
Also, air route surveillance and air traffic control radars operate throughout the United States. The locations of these radars are accurately known, as are their mechanical and electronic characteristics. These radars are equipped with what is known as a "secondary radar" which operates in synchronization with the primary radar and functions as an interrogator to airborne transponders. The purpose of the interrogator/transponder system is to provide a capability to track aircraft at ranges, under conditions, and to an accuracy which would otherwise be impractical, if not impossible. The interrogator will trigger airborne transponders anywhere within radar line of sight of the interrogating radar. The responses from interrogated aircraft can be detected and accurately displayed at air traffic controller positions even when range, meteorological conditions, traffic density, or electrical interference degrade the performance of the primary, or skintrack radar.
Airborne transponders are carried by all airline and military aircraft, and by nearly all commercial aircraft. Transponders are also carried by a high (and growing) percentage of private aircraft. Use of these transponders is mandatory in many flight conditions and in certain designated high traffic-density areas (Terminal Control Areas, or TCA). The transponder provides primarily a recognizable target to the interrogating radar. In addition, certain transponders provide the altitude data of the transponding aircraft, and other information.
There is a substantial need for a system which utilizes these existing facilities for aircraft collision avoidance, without requiring the use of an excessive amount of additional equipment.
The following prior U.S. patents were found in a preliminary search and are believed to be of interest in showing the present general state of the art:
Van Valkenburgh, U.S. Pat. No. 2,837,738 PA1 Harvey, U.S. Pat. No. 2,842,764 PA1 Campbell et al, U.S. Pat. No. 2,933,726 PA1 Miner, U.S. Pat. No. 2,969,539 PA1 Alexander et al, U.S. Pat. No. 3,025,514 PA1 Perkinson et al, U.S. Pat. No. 3,250,896 PA1 Stansbury, U.S. Pat. No. 3,310,806 PA1 Stansbury, U.S. Pat. No. 3,469,079 PA1 Carter, U.S. Pat. No. 3,808,598